The invention relates generally to antennas and more particularly to antennas for providing polarized radiation designed based on holographic principles.
Several new applications are emerging at Ka-band frequencies (26-40 GHz.) including Local Multi-point Communication/Distribution Systems (LMCS/LMDS) and advanced satellite communications systems (SATCOM). These systems must be capable of delivering high-bandwidth multimedia signals, therefore providing simultaneous services such as voice, fax, high-speed Internet access, videoconferencing, and many others. There is a strong requirement for these applications to increase channel capacity and to reduce interference due to impairments from obstacles such as rain, terrain variations, trees and buildings. A method of accomplishing this is through the use of dual and circular-polarized systems. Since these types of services are targeted for both business and home users, low-cost fixed or mobile user terminals are desirable to encourage widespread implementation. If the terminal is to be mounted on a house or on top of a car, then a low profile and lightweight design is preferred.
Traditionally when the above requirements are specified. high gain reflectors or reflect arrays are used due to their relatively high efficiency (up to 50%). For such a system, the feed is necessarily capable of supporting either dual or circular-polarization. This is also true with conventional lenses, which have efficiencies similar to those of reflectors. As well, passive microstrip phased arrays are employed for their low profile and multi-beam capabilities; however, passive microstrip phased arrays have less radiation efficiency. Reflectors and reflect arrays have a high profile, a heavy weight, and in many cases suffer from feed aperture blockage. An offset reflector configuration or a lens is often used to eliminate aperture blockage, resulting in increase size and/or complexity. Also, these systems require complex feeds to generate dual and circular-polarization in radiation emitted or reflected therefrom. Finally, these systems have a limited beam scan range for multi-beam applications unless a complicated surface shaping is applied. The disadvantage of low profile printed phased arrays is the high feed losses, which become significantly large at Ka-band and degrade the radiation efficiency. To compensate for these losses, amplifiers are added in the feed network, which increases the complexity and the cost, and may introduce additional problems such as oscillation and overheating. Presently active phased array antenna technology is being researched, particularly to achieve better dual and circular-polarization where isolation between polarization directions is often a limiting factor.
K. Iizuka et al. proposed a traveling wave antenna constructed based on holographic techniques in xe2x80x9cVolume-Type Holographic Antennaxe2x80x9d, IEEE Transactions of Antennas and Propagation, vol. AP-23, November 1975, pp. 807-810. Effectively, a plurality of printed arcs on a substrate are irradiated from a source. The radiation is scattered in both directions. The use of a second similar printed substrate allows for radiation scattered behind the substrate to be scattered forward again in order to increase overall directionality and efficiency. In this cases the antenna disclosed therein provides a radiation pattern that is polarized in one direction.
Dual-polarized and circular-polarized traveling-wave antennas are known. Most of these antennas, such as those proposed in W. J. Getsinger, xe2x80x9cElliptically Polarized Leaky-Wave Arrayxe2x80x9d, IRE Transactions on Antennas and Propagation, vol. AP-10, March 1962, pp. 165-171 and A. Chan and M. Kharadly, xe2x80x9cHigh Gain, Dual Frequency, Dual Polarization, Low Profile Antenna Design for Millimeter-Wave Communication Systemsxe2x80x9d, Tenth International Conference on Antenna and Propagation, Apr. 14-17, 1997, Edinburgh, UK, pp. 1.390-1.393. are rectangular waveguiding structures with open apertures on one wall of the guide. Since these antennas are fast-wave structures, the practical radiated beam peak angle range is 10xc2x0xe2x89xa6xcex80xe2x89xa685xc2x0 (xcex80 is shown in FIG. 1). Therefore, broadside radiation (xcex80=90xc2x0) and end-fire radiation (xcex80=0xc2x0) is difficult to obtain with fast-wave antennas (also known as leaky-wave antennas).
The cylindrical DR rod antenna fed by a short helix to generate circular polarization described in H. T. Hui, Y. A. Ho, and E. K. N. Yung, xe2x80x9cA Cylindrical DR Rod Antenna Fed by Short Helixxe2x80x9d, IEEE AP-S International Symposium, Jul. 21-26, 1996, Baltimore, Md., USA, vol. 3, pp. 1946-1949 is another interesting concept. However, the antenna only radiates end-fire because of the surface-wave mode it supports. There is also circularly polarized microstrip antennas such as Rampart-line and Chain traveling-wave arrays, but at millimeter-wave frequencies, these structures are very lossy. Another circularly polarized traveling-wave antenna, described in C. S. Lee and V. Nalbandian, xe2x80x9cCircularly Polarized Traveling-Wave Microstrip Antennaxe2x80x9d, IEEE AP-S International Symposium, Jun. 21-26, 1998, Atlanta, Ga., USA, vol. 2, pp. 908-911 consists of a double-layer probe-fed microstrip half-circle that behaves as a leaky-wave transmission line. For high gain applications, an array of such elements requires a complex feed structure since each half-circle requires a probe. Also, phasing each element to scan the beam off broadside might significantly degrade the axial ratio at the beam peak due to the fix beam characteristic of the single element.
It would be advantageous to provide an antenna design based on holographic techniques that supports dual polarization and circular polarization.
It would be advantageous to provide a low profile light weight inexpensive antenna design for use in high frequency applications.
It would also be advantageous to provide a travelling wave antenna supporting dual or circular polarizations without requiring complex electronic circuitry.
In order to overcome these and other shortcomings of the prior art, it is an object of the present invention to provide an antenna that is capable of providing dual linear or circularly polarized radiation that is low profile and inexpensive to manufacture.
The dual and circular-polarized traveling-wave antennas of the invention overcome the above limitations by transforming a surface-wave mode to a leaky-wave mode or radiating-mode using a quasi-periodic grating structure or discontinuities. The antennas allow a radiated beam peak angle range of 0xc2x0xe2x89xa6xcex80xe2x89xa6180xc2x0 (xcex80=180xc2x0 is called back-fire radiation) and only require a single linear-polarized feed to generate the pattern. Losses associated with microstrip antennas are minimal with the dual and circular-polarized traveling-wave antennas since only a little amount of copper is used.
In accordance with the invention there is provided an antenna comprising: a dielectric having first scattering elements disposed thereon in a first interference pattern for scattering radiation provided along first predetermined feed direction and second scattering elements disposed thereon in a second other interference pattern orthogonal to the first interference pattern for scattering radiation provided along a second predetermined feed direction, wherein during use a substantial amount of isolation exists between the radiation along the first feed and the second feed.
Preferably, the antenna is provided energy, fed, in two orthogonal directions from each of two feeds. Advantageously, the provided energy need not be polarized for the radiated energy to be polarized in a predetermined fashion. As such, the need for complex feed circuitry is obviated.
According to another embodiment the antenna comprises a first dielectric substrate comprising a plurality of first groups of linear scattering elements, each first group disposed in an arc, different first groups disposed along different arcs, linear scattering elements within the first groups each for scattering radiation with a predetermined polarization. Preferably, each group comprises a plurality of linear scattering elements forming a broken arc, broken in that the linear elements are each positioned on the arc to approximate the arc but, since they are linear, the resulting form is not a continuous arc.
In an embodiment the first dielectric substrate includes a plurality of second groups of linear scattering elements, each second group disposed along different arcs, the second groups for scattering radiation with a polarization orthogonal to the predetermined polarization.
In another embodiment the antenna comprises a dielectric substrate having scattering elements disposed thereon in an approximate interference pattern, the scatting elements parallel to a single plane for scattering radiation provided thereto from a feed disposed for radiating a traveling wave along the substrate into a radiation field having a single linear polarization.
In yet another embodiment the antenna comprises a first dielectric substrate having scattering elements disposed thereon in an interference pattern, the scatting elements parallel to a single plane for scattering radiation provided thereto into a radiation field having a single linear polarization and a first feed disposed to irradiate the dielectric for producing a linearly polarized radiation pattern scattered therefrom.
Antennas according to the invention combine the advantages of low-profile printed technology with an unconstrained feed to avoid excessive losses associated with conventional microstrip phased array feed networks. By varying the destructive interference pattern etched on a very thin dielectric slab it is also possible to design low-cost dual and circular-polarized traveling-wave antennas. Another interesting feature of these antennas is that optionally the feed is in the same plane as the dielectric slab, making the structure almost flat and preventing feed aperture blockages. Also, for designs for emitting circularly polarised radiation, these antennas to optionally use a simple linear polarized feed instead of a more complex circular-polarized feed required with conventional reflectors or lenses.